In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two closed (endless) wires, fabrics or belts which press the web, one from above and the other one from below, against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular when the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single closed (endless) drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls arranged between the drying cylinders, the web remains at the side of the outside curve and is subjected to negative pressure as it runs over the reversing cylinders or rolls in order to maintain the web on the wire. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
In so-called normal groups with single-wire draw, known in the prior art, the heated drying cylinders are placed in an upper row and the reversing cylinders or rolls are placed in a lower row below the upper row of drying cylinders, which rows are typically horizontal and parallel to one another. In the following, when the term "normal (dryer) group" is used, what is meant is expressly groups with single-wire draw in multi-cylinder dryers of the type mentioned above.
It is known to those skilled in the art that if paper is dried one-sidedly or unequalsidedly, the result is a tendency of curling of the sheet. For example, when paper is dried by means of normal groups with single-wire draw from the side of its bottom face only, the drying is asymmetric and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, when viewed from above.
As known in the prior art, the tendency of curling of paper (or the tendency to curl) is already affected in connection with the web formation, in particular at the sheet formation stage by means of the selection of the difference in speed between the slice jet and the wire, by means of the selection of the former and its mode of running and by means of other running parameters. For example, in the case of copying paper, by means of unequalsidedness of drying in the afterdryer, a suitable initial curl form is regulated for the sheet in order that the curling of the paper after one-sided or double-sided copying could be optimized. In the case of copying paper, the reactivity of curling, i.e., the extent to which curling occurs per unit of change in moisture content, is affected to a greater extent by means of a multi-layer structure of the paper, which is produced in connection with the web formation in the wet end.
With respect to the prior art related to the present invention, reference is made to the current assignee's Finnish Patent No. 91,900 (corresponding to U.S. Pat. No. 5,416,980 incorporated by reference herein), in which a method is described in the dryer section of a paper machine in particular for reducing the tendency of curling of paper. In this method, the paper web is dried by means of drying cylinders against whose heated faces the paper web is pressed by a drying wire. In the dryer section, groups of drying cylinders are used in which twin-wire draw and/or single-wire draw is/are applied. In this method, it has been considered advantageous that in the dryer section, substantially across the entire width of the paper web, hot water or steam is fed to relax strains or tensions that arise or tend to arise in the fiber mesh in the paper web in, or substantially directly after, the area of formation of the strains or tensions.
In the current assignee's Finnish Patent Application No. 963734 (corresponding to U.S. provisional patent application Ser. No. 60/030,693), a method is described for drying a surface-treated paper web or equivalent in an after-dryer of a paper machine as well as a dryer section of a paper machine for applying the method. With a view toward compensating for a tendency of curling of the paper web, in the after-dryer, the paper web is dried in a dryer group/groups making use of a normal single-wire draw. In connection with or after the drying, the paper web is treated by means of at least one device in order to compensate for a tendency of curling of the paper web, which devices are, for example, a steam box, a blower unit, a moistening device, and/or a soft calender.
On the other hand, in the current assignee's Finnish Laid-open Publication No. 98,387 (corresponding to U.S. patent application Ser. No. 08/705,059), a method is described for manufacturing a paper to be surface-treated, in particular fine paper, as well as a dry end of a paper machine for applying the method. The paper web, which has been dewatered by pressing, is dried in a forward dryer section in which drying energy is applied to the paper web over the entire length of the forward dryer section asymmetrically in the z-direction from the side of the bottom face of the web. This stage is carried out by means of a number of successive groups with single-wire draw that are open towards the bottom while the web is carried on support of a drying wire. In this manner, shrinkage of the web both in the machine direction and in the cross direction is substantially prevented, which shrinkage tends to occur with an increase in the dry solids content. In connection with a web break, the paper broke is removed from below the dryer groups that are open towards the bottom onto a broke conveyor placed underneath substantially by the effect of gravity. The paper web, which has a tendency of curling because of the asymmetric forward drying, then is passed into an after-dryer in which it is after-treated while, at the same time, moistened and/or plastically worked, so that the tendencies of curl that arose in it in the forward drying stage are eliminated. For example, the after-dryer may include groups with twin-wire draw and regulation of steam as well as steam boxes that have been arranged in view of controlling the curl, as well as infra and airborne web dryers.
In the current assignee's Finnish Patent Application 964830 (corresponding to U.S. Provisional Patent Application Ser. No. 60/032,405), a method is described for drying paper as well as a dry end of a paper machine. The method for drying of paper comprises the following steps:
the paper web to be dried is passed from the press section into a forward dryer section, in which the paper web is dried from the side of its bottom face in dryer groups that apply a normal single-wire draw, the forward dryer section comprises exclusively single-wire groups with normal single-wire draw, PA1 the paper web is passed from the forward dryer section into a finishing section in which the paper web is coated/surface-sized by means of coating/surface-sizing equipment, PA1 the paper web is passed from the finishing section to be dried in an after-dryer section in which the paper web is dried in at least one dryer group that applies normal single-wire draw, PA1 after the after-dryer section, the paper web is calendered in a calender and passed to a reeling station in which the paper web is reeled into a machine reel, and PA1 curling of the paper web is controlled by means of curl control elements and/or by means of assemblies and combinations formed out of such elements in the area of the forward dryer section and/or the finishing section.
The dry end of a paper machine described in FI 964830 comprises a forward dryer section, a finishing section which comprises a coating/surface-sizing apparatus, an after-dryer, a calender and a reeling station. The dry end of the paper machine comprises curl control elements and/or assemblies and combinations formed out of such curl control elements in view of controlling the curling of the paper web in the area of the forward dryer section and/or the finishing section. The elements for controlling the curling comprise, among other things, means for blowing hot moist air through the wire in the forward dryer section, steam boxes employed in the after-dryer, a combination in which steam-treatment by means of a steam box is combined with a cooling cylinder, a lower support belt or support wire in the after-dryer, twin-wire groups employed in the after-dryer, means for through-drying through the wire in connection with at least one cylinder in the after-dryer, a pre-selected ratio of cylinder diameters, means for spraying water against the web in the after-dryer, infrared boxes arranged before the calender, means for transferring moist air from the forward dryer to the after-dryer to be blown against the web, and mechanical working of the web by means of a spreader bar.
With respect to the prior art, reference is also made to U.S. Pat. No. 5,557,860, in which a dryer section is described which includes dryer groups that make use of a normal single-wire draw and a moistening device arranged after the dryer groups by whose means the curl is controlled.
From the prior art, it is also known to moisten the air that surrounds a winding device, in which case drying and shrinkage of the paper in the winding device are prevented. An excessively low relative humidity of the air surrounding the winding device results in uncontrolled drying and shrinkage of the paper, which again causes cutting off of the paper at the knife blades and makes the slitting more difficult and also makes the formation of splices more difficult. Under excessively dry conditions, there may also be difficulties in meeting the requirements of dimensional precision imposed on a roll. This prior art moistening system comprises moistening nozzles and air devices installed below the winding device. By means of these devices, the air that rises to the winding device is moistened and directed at the knife blades under controlled conditions. If necessary, the moisture level can be adjusted continuously. Thus, in this prior art construction, control of the curl of paper is not aimed at.
In the prior art constructions described above, the devices for the regulation of curl and the other, corresponding arrangements are placed in the dry end of the paper machine before the calender. It has been assumed that, if curl regulation operations are carried out after the calender, they have a detrimental effect on the surface properties of the paper. However, it is a drawback of a curl regulation device, such as a steam box or a moistening device, placed before the calender that, at the same time, the calendering result is affected to a great extent by the operation of the curl regulation device, i.e., the smoothness of the faces and the bulk of the paper is altered. This quite often prevents full control of the curl. On the other hand, the efficiency of a moistening/steam-treatment device suffers from the high temperature of the web. Devices arranged in the interior of the hood of the dryer section must also be designed in consideration of the hot and moist environment. Also possible servicing work must be carried out at times of standstills.
The curl regulation devices arranged before the calender must be placed near the end of the dryer section, at which time the temperature of the web and of the surrounding air is highest. In such a case, for example, the use of a steam box is not so efficient (without cooling of the web), for steam does not condense into the hot web (having a temperature of 70.degree. C. to 80.degree. C.) and thus, does not equalize the 2-way moisture profile. In order to cool the web, for example cooling cylinders are employed and as a result, the length of the dryer section is increased, which is not economically advisable. Also, at the end of the dryer section, owing to the hot web, larger quantities of steam or water are needed for correcting the 2-way moisture profile.